Behavioral advertising, as such tracking is known, is becoming increasingly common on the web. Advertisers use cookies to follow you around the web, tracking which sites you visit, what you buy and even, in the case of mobile browsers, where you go. The U.S. Federal Trade Commission has already outlined a Do Not Track mechanism (PDF link), which would work much like the FTC’s Do Not Call list, offering a way to opt-out of online tracking.

The proposed do-not-track HTTP header is one of several ways Mozilla plans to implement the FTC’s suggestions. While the header idea has been around for a while — the Do Not Track Firefox add-on from the Stanford Law School is one example — currently most online opt-out schemes use cookies to set user preferences. Mozilla believes “the header-based approach has the potential to be better for the web in the long run because it is a clearer and more universal opt-out mechanism than cookies or blacklists.”

While the new header is just a proposal at the moment, Mozilla already has some code ready and is considering adding the feature to future versions of Firefox. The current plan is to create a new preferences option that would allow you to opt-out from tracking. Check the box in the preferences and Firefox will start sending the do-not-track header each time you request a new page.

Interestingly, the header Mozilla proposes is not the same as the “X-Do-Not-Track” proposal, which is already implemented in Firefox add-ons NoScript and Adblock Plus. For more details on how Mozilla’s new HTTP header will work, see Mozilla developer Sid Stamm’s blog post.

Like Mozilla’s proposed privacy icons, the problem with the new header is getting third-party ad sites to obey it. Mozilla calls it a “chicken and egg” problem and hopes to jumpstart the idea by including the header in future releases of Firefox. At that point it would be up to third party websites to support the header and, as Mozilla puts it, “honor people’s privacy choices.”

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